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Sensors and detectors
How to use sensor and detectors (in robotics)
RACE PROJECT
VIGO (SPAIN) September 26-29, 2012
Definitions (1)
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Sensor: a device for sensing a physical variable of a physical system or an
environment
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A sensor is in most cases associated with electronic circuits (converter) to
generate a conditioned, normalized, amplified electric signal
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A sensor plus a converter form a transducer, a device which transforms
energy from one type to another (in this specific case electric energy)
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Sometimes the sensor reacts by generating an electric signal itself, so it
can be considered a transducer
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Definitions of sensors and transducers do not always agree, so the word
“sensor” alone might be enough to indicate these types of devices
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Sensors are what the robots need to know the world …
◦ from http://www.robotshop.com/sensors.html
Classifications 1 (physical characteristic)
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Primary (sensors/transducers)
◦ Temperature
◦ Light (photoelettric)
◦ Strain gauge / Mechanical tension
◦ Magnetic field
◦ Displacement (potenziometers)
◦ …
 Secondary
(sensors/transducers)
◦ Force, acceleration, pressure (based on strain gauge)
◦ Displacement (based on photoelettric, magnetic field, capacitance, …)
◦ Speed (based on displacement and time)
Classifications 2 (output electric signal)
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Analog
◦ The output is an electric signal which varies continuously according to the
variations of the physical variables beeing measured
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Digital
◦ The output is an electric pulse signal which can assume only two values:
logical 0 and 1. The frequency or the code associated with the pulse
sequence carries the information about the physical variables beeing
measured
Classifications 3 (energetic behavior)
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Active
◦ They provide an electric signal which can be directly processed without
further consumption of energy: for example the photovoltaic cells and
termocouples
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Passive
◦ The require an electrical generator in order to transduce the physical
variable in an electric signal: for example the potentiometer
Specifications (static and dynamic parameters)
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Transfer function (transcaratteristic)
Monotone function
Linearity
Offset
Operation range
Hysteresis
Sensitivity
Resolution
Repeatibility
Stability
Response time (time costant e bandwidth)
Input and output impedance
Specifications (parametri caratteristici statici e dinamici)
Monotone function
Linearity
Sensitivity
Sensors (in this presentation)
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Thermoresistance, thermistor, PN junction, integrated sensor
(temperature)
Photoresistor, photodiode, phototransistor (light)
Tachometer (angolar speed)
Encoder (different tipes for displacement and speed)
Temperature - Thermoresistance (1)
Metallic conductors with a known “resistance vs temperature
caracteristic curve”.
The basic physical principle of these devices is that the electric
conductivity (resistivity) decreases (increases) as the temperature
increases. This holds true for materials like platinum, nickel, copper.
The value of T is in 0C
If β and γ are small compared to the value of α this relation can be
considered linear (for example platinum)
If not it might be necessary to perform a linearization (for example for
nickel and copper for temperatures above 1000C)
Thermoresistance have a low sensitivity
Temperature - Thermoresistance (2)
Example of a temperature monitoring system from 00C to 3000C, output tension
between 0V and 10 V, based on PT100.
VR
V1
Temperature - Thermistor NTC
Unipolar semiconductor material
The basic physical principle of these devices is that the electric conductivity
(resistivity) of the pure semiconductor material increases (decreases) as the
temperature increases.
NTC (Negative Temperature Coefficient); T is in 0K
Thermistors are very sensitive, but R is strongly non linear with respect to T.
With highly doped semicobductor material it is possible to obtain PTC type (Positive
Temperature Coefficient) thermistors
Temperature - PN junction
In a direct bias PN junction with costant current the direct diode tension
decreases by 2,5 mV with the increase of 10C of the temperature.
The exact value of the tension for a given temperature depends upon the
value of the costant current of the diode.
A small signal junction diode like 1N914 or 1N4148 can be easily used as a
temperature sensor.
Good time response
Calibration is required
Temperature - IC AD590 (1)
Integrated sensor:
Devices which embed the sensor plus the circuits to normalize, linearize,
amplify the signal, in other words a “transducer”.
The integrated T sensors are based on the linear dipendence between VD e T of the
direct bias costant current diode (see prevous page). The junction is the BE junction of
a BJT transistor
AD590: high impedence current generatori
T is in 0K; K is μA/0K
The generated current is directly proportional to the absolute T value
It can be located far from the measurement instrument (it works with current) and it
is not very sensitiv to noise
It can show scale and offset errors
The output current signal is converted to tensiom through a resistance plus a I/V
converter (for example based on OpAmp)
Temperature - IC AD590 (2)
Light sensor – Photoresistor (1)
Devices in which the information associated to light is
converted in variation of resistance: resistivity decreases
(conductivity increases) as the light increases
 Made with N type semiconductore materiale (not a PN
junction)
 Thoughness, low priced, sensitivity
 They can dissipatehigh values of power (for example to
control relays)
 Limited bandwidth
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Light sensor – Photoresistor (2)
Light sensor – Photodiode (1)
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When a reversed biased PN junction is illuminated the
total reverse current is given by the sum of the typical
revers current plus a component proportional to the
luminous flux
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These devices are very fast and are highly used as
detectors in telecommunication systems based on fiber
optics
Light sensor – Photodiode (2)
Speed -Tachometer generator (1)
The tachometer generator (dynamo) is a small generator that produces an
output voltage that is very accurately determined by its operating speed
Tachometer generator (2)
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Simulation file
Speed - Incremental encoder (1)
Device which measures the angolar displacement of a shaft in order to get informations
about the angular speed (for example of a motor)
It is made by a rotary disc and a Led/fototransistor system.
On the circumference of the disc a set of holes has been set all at the same distance on
from the other.
When the disc rotates the light beam either is interrupted (no hole and bjt in cut-off
mode) or goes through from the led to the phototransistor (precence of the hole and bjt
in saturation mode).
The phototransistor generates a train pulse, one pulse for each hole crossed by the light
beam.
From the number of pulses it is possible to determine the angular displacement and, in
relation to time, the angular speed
With the incremental encoder it is possible to measure the speed, but it is not possible to
determine the rotational direction
Speed - Incremental encoder (2)
Speed - Incremental encoder (2)
INSERIRE LA FIGURA
Speed - Incremental encoder (3)
Two/three phases incremental encoder to determine the rotational direction
Speed - Incremental encoder (4)
Two/three phases incremental encoder
Speed – Absolute encoder (1)
Used to determine the shaft angular position.
Each combination of holes is coded so to provide the angular position of the
disc.
Normally the Gray code is used in order to prevent the transmission of
errors
With simple combinatory Exor circuit it is possible to convert Gray code in
natual binary code
Motor control with encoder (1)
Analog control system
Motor control with encoder (2)
Digital control system
Sensors (Society of robots)
Sensors specific for robotics: Society of
robots
 http://www.societyofrobots.com/sensors.s
html
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Sensors in robotics & applications
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Accelerometer
Color Sensors
Digital Compass
Encoder (Slot, Rotary, Linear)
Infrared Emitter/Detector
Load and Torque Sensors
Mercury Tilt Switch
Photoresistor
Robot Computer Vision
SharpIR Rangefinder
Sonar
Tactile Bumper Switch
Sensors in robotics & applications
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Accelerometer
Color Sensors
Digital Compass
Encoder (Slot, Rotary, Linear)
Infrared Emitter/Detector
Load and Torque Sensors
Mercury Tilt Switch
Photoresistor
Robot Computer Vision
SharpIR Rangefinder
Sonar
Tactile Bumper Switch
Robot for the contest
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Ideas for the final meeting